NTSC Display Basics

The final graphics product of the 3DO system is a signal sent to an NTSC television display. Because all of the graphics hardware within the 3DO system is aimed at this goal, the NTSC standard is the first place to start for understanding 3DO graphics. This chapter won't try to discuss all of the workings of an NTSC display-that's a topic for a book devoted to NTSC. You'll find just enough information here to understand how the 3DO graphics components time their operations and format the image they send to the display.

If you're an NTSC pro and already know the basics, you can skip the first part of this section and turn to 3DO NTSC Graphics.

Image Scanning

Any image displayed on an NTSC television set is rasterized; that is, it's broken down into a set of horizontal scan lines. The NTSC-TV displays a rasterized image by drawing the image one line at a time, using a set of three electron beams: one for red, one for green, and one for blue. Those three beams sweep horizontally across the phosphors of the display tube, changing their intensity as they go, to change colors along the length of the scan line.

An NTSC image is displayed with 525 scan lines of which the first 40 are invisible-used in old sets to warm up the electron guns for the new image. The 3DO system uses 480 of the remaining 485 scan lines to draw its images on the screen; 480 because the number works out well with a 320-by-240 resolution that is standard in computer graphics today.

Frame Rates

Each full image scanned on the NTSC display is a frame. The NTSC standard provides for a full frame to be drawn 30 times a second. By presenting a series of frames on the display, a television set provides the illusion of motion. Unfortunately, the rate of 30 frames per second falls below the rate of 50 to 55 images per second necessary to trick the human eye into seeing flicker-free motion.

To get rid of the flicker, NTSC divides each set of frame scan lines in half; each half-frame is called a field. The first field, called the odd field, contains all the odd-numbered scan lines (which are numbered starting from the top of the screen). The second field, called the even field, contains all the even-numbered scan lines. The two fields are interlaced as shown in Figure 1 to create a full frame.

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Figure 1: NTSC fields.

An NTSC display draws fields on the screen at a rate of 60 fields per second, alternating between odd and even fields. The display rate surpasses 50 to 55 images per second, so the human eye sees flicker-free motion. And because the odd and even fields interlace to provide a full frame, the display still presents 30 frames per second.

The Scan Pattern

When an NTSC's electron beams scan the phosphors of the display tube, they follow a set scan pattern shown in Figure 2. They scan the first line of a field, moving left to right, as they change intensity to change color and intensity on the screen. At the end of the first line, they turn off, their aim moves left to start at the beginning of the next line, then they turn on again and scan another line. The interval between the end of one scan line and the beginning of the next is called the horizontal blank, a period when nothing is beamed onto the display screen.

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Figure 2: NTSC scan pattern.

When the electron beams reach the bottom of the field (which can be either the odd or even field depending on the timing), they turn off once again and move to the top left of the next field's first scan line, where they start the scanning process over. The interval between the end of one field and the beginning of the next is called the vertical blank, another period when nothing is beamed onto the display screen.

Notice that scan lines aren't perfectly horizontal in Figure 2, but slightly slanted down as they move left. Notice too that the 525 frame lines are divided exactly in half, so each field gets 262.5 scan lines-hence the half-line in each frame in the figure. Although the figure shows the true physical nature of NTSC scan lines, for all practical purposes you can consider each scan line as perfectly horizontal and disregard half-lines; they're invisible in each frame and aren't used for drawing. Think of each field created by 3DO hardware as a set of 240 perfectly horizontal and full scan lines.

3DO NTSC Graphics

The 3DO graphics hardware takes best advantage of the NTSC standard by providing discrete color pixels of 320-by-240 resolution, NTSC's highest effective color resolution. 3DO graphics hardware also provides subpixels of 640 by 480, NTSC's highest effective intensity resolution (the electron guns in an NTSC display can change intensity twice as quickly as they can change color). As you'll read later, the 3DO hardware can divide up the colored pixels into subpixels and then smoothly average colors between the subpixels so the final result is a clear NTSC signal carrying a 640-by-480 image. That image is displayed in 480 scan lines, each of which can change in 640 discrete increments along the line.

The 640 lines in a typical 3DO frame are divided into odd and even fields, and alternately sent to the NTSC display. Although the resulting fields are slightly different (due to the smooth color grading), they merge together to create a single frame with 320-by-240 color resolution and 640-by-480 contrast resolution.

Stereoscopic Display

To display stereoscopic images on an NTSC screen, 3DO hardware uses a peripheral viewing device: electronically controlled glasses with LCD shutters in each lens. Those shutters alternately turn on and off 60 times per second. They permit the left eye to see only even fields and the right eye to see only odd fields. The 3DO graphics hardware presents slightly different images for each field by switching between two frame buffers: one for the odd field, the other for the even field. If the images in each buffer are carefully constructed in parallax, the viewer sees a single image in apparent 3-D.

Video Timing

Much of the 3DO hardware's processing time is spent preparing an image for NTSC presentation; it's crucial that graphics processes are timed to coincide with the presentation rate of frames, fields within the frames, and scan lines within the fields. To make that possible, the 3DO hardware provides signals at the beginning of each horizontal and vertical blank. Some processes, such as those that change available colors for an image, use horizontal blank signals so they can work to change conditions between one scan line and the next. Other processes, such as those that refresh background images, use vertical blank signals to work between fields or between frames.